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Leidenfrost Effect Puts Perpetual Bounce into Hydrogel Beads

Tuesday, March 29, 2016

Not all great ideas come in the shower. Sometimes inspiration strikes when you're whipping up a stack of pancakes.

At least that's what happened to the man who filmed the video above when he paused his cooking to perform an impromptu science experiment: dropping a handful of hydrogel beads -- small orbs that absorb water and can be used instead of soil for houseplants -- into a hot skillet.

Because, well, why not?

Last November, the amateur scientist posted a YouTube video of the beads, which are also used in gardening or landscaping to help retain moisture in the soil, bouncing around the pan like too many kids on a trampoline. And then the reactions and speculation as to why the beads acted as they did began to simmer.

In comments, some YouTube users made connections to familiar and fundamental physics. Sergio Romero wrote that the video "looks like a representation of atoms bouncing around when in a gas form."

Some raised other concerns. ZOMG Gaming! suggested "This is water cruelty you should be ashamed im calling the internet police." Brent Allen wrote, "I think there's something wrong with your grapes." (Their comments are presented as they appeared.)

A number of people, however, pointed to something called the Leidenfrost effect.

Even if you’ve never heard of the effect, you've probably at least seen it in your kitchen. It occurs when a droplet of liquid, say water, comes in contact with a hot surface, say a frying pan, raised to a certain temperature, and the droplet creates some vapor between it and the pan. As a result, the droplet doesn't actually touch the surface. Instead, it's levitated, skittering around the pan on its vapor cushion rather than evaporating quickly as expected.

When Scott Waitukaitis, a postdoctoral researcher and physicist at Leiden University in the Netherlands, saw the video, he and his colleagues decided to replicate the experiment in the lab, teasing out the physics behind the beads' strange reaction.

They found that when they dropped a hydrogel bead on a hotplate at roughly room temperature, it did "almost exactly what you’d expect for a bouncy ball," said Waitukaitis, during a presentation last week at a meeting of the American Physical Society in Baltimore. "That is, it bounces, and every time it gets a little bit lower and a little bit lower and a little bit lower until it comes to rest."

But when the researchers repeated the experiment with the hotplate heated to 437 degrees Fahrenheit, things got interesting.

Although at first the ball bounced a bit lower and lower – just as it did on the first plate – after a while the bounce height stopped decreasing and stayed constant at a height of about 2 cm.

And when it comes to this hydrogel bead on a hot surface -- it can't stop, won't stop. Waitukaitis and a colleague started one of the balls bouncing in the lab one day, and "lo and behold, 10 minutes later it was still going," he said.

The scientists used a high-speed camera to film the experiments and a microphone to capture the high-pitched squeaks the beads produced as they came in contact with the hot surface.

But does the Leidenfrost effect explain what's happening?

It's "related to the Leidenfrost effect, but it's a new aspect . . . the big thing is that this is Leidenfrost with an interplay between elasticity and vapor. So we get all these new dynamics from that," said Waitukaitis.

In his presentation, Waitukaitis stressed that the research is preliminary. (He also highlighted the YouTube comments used above.) But here's what he explained at the meeting: As the bead is dropped into the pan, vapor boils off its surface and gets shot down into little cavities in the frying pan’s surface. Because the bead is squishy, it deforms as it hits the pan, momentarily trapping the vapor in the tiny spaces -- unlike in the regular Leidenfrost effect where the vapor can escape out the sides of the droplet. As a result, pressure builds up underneath the ball, giving it a little extra energy when it bounces.

"This is where the story is right now," said Waitukaitis. "[Our model] also makes the strange prediction that if I throw something softer on there, it will actually bounce higher. That's what we want to test next."